The effect of bone loss on rod-like and plate-like trabeculae in the cancellous bone of the mandibular condyle

Three-dimensional radiological anatomy of condyle trabecular bone based on a Volume-of-Interest analysis

OBJECTIVES

Three-dimensional radiological anatomic characteristics of condyle trabeculae was obtained quantitatively based on a volume-of-interest (VOI) analysis.

METHODS

Nine human mandibular condyle specimens were scanned by micro-computed tomography (micro-CT). A total of 34 VOIs were selected from each condyle specimen, which were divided into six layers and four parts to analyze the morphological characteristics of trabeculae based on cylindrical VOIs with a diameter and height of 2 mm. One-way analysis of variance was used to compare the regional differences of morphological parameters among each layer and part.

RESULTS

Values for bone mineral density, bone volume/total volume, trabecular thickness, and trabecular bone number were greater in the anterior part compared with the posterior part; and the lateral part was larger than the medial part in the first, second, and third layers, while the medial part was larger in the fourth and fifth layers; these values in the first and sixth layers were much larger, while those in the third and fourth layers were smaller. Bone surface area/bone volume, trabecular spacing, and trabecular bone pattern factor were larger in the posterior part than in the anterior part; and the lateral part was larger than the medial part in the fourth and fifth layers, while the medial part was larger in the first and second layers.

CONCLUSIONS

The morphological distribution of VOIs was anisotropic within trabecular bone of human mandibular condyles. The upper and lower ends of trabecular bone were much more compact, with higher bone density, trabecular thickness, and trabecular number than in the middle layers.

Degenerations in Global Morphometry of Cancellous Bone in Rheumatoid Arthritis, Osteoarthritis and Osteoporosis of Femoral Heads are Similar but More Severe than in Ageing Controls

We have recently revealed significant differences in microarchitectural properties (i.e. global and local morphometries) and mechanical properties between rheumatoid arthritis (RA), osteoarthritis (OA) and osteoporosis (OP) in cancellous bones. This study compared these properties with those of ageing controls by matching bone volume fraction (BV/TV), the most important determinant for bones' mechanical properties, to investigate whether these bones have similar properties and degenerative potentials. RA, OA and OP femoral heads were harvested from patients undergoing total hip replacement surgery. The selected patients were matched by similar cancellous bone BV/TV, with seven patients in each group. Four samples were prepared from each femoral head and scanned with micro-CT to quantify microarchitectural properties and compression tested to determine mechanical properties. In terms of global morphometry, no significant differences were observed between these diseased bones. In terms of local morphometry, the number of plates in the RA group was significantly greater than that of the OP and control groups. Plate volume density in the RA group was significantly greater than in the control group. Interestingly, the ultimate stresses in the three diseased groups were 77% to 195% lower than in the control group (p < 0.001). Degenerations of global morphometry of cancellous bones in these diseased femoral heads are similar but more severe than in ageing controls matched by BV/TV, as evidenced by pronounced reduction in bone strength. This phenomenon suggests that some local morphometric parameters, along with other factors, such as abnormal collagen, mineralisation, erosion and microdamage, may contribute to further compromising mechanical properties.

Trabecular bone porosity and pore size distribution in osteoporotic patients - A low field nuclear magnetic resonance and microcomputed tomography investigation

The study of bone morphology is of great importance as bone morphology is influenced by factors such as age and underlying comorbidities and is associated with bone mechanical properties and fracture risk. Standard diagnostic techniques used in bone disease, such as Dual-Energy X-ray absorptiometry and ultrasonography do not provide qualitative and quantitative morphological information. In recent years, techniques such as High Resolution Computed Tomography (HR-CT), micro- CT, Magnetic Resonance Imaging (MRI), and Low Field Nuclear Magnetic Resonance (LF-NMR) have been developed for the study of bone structure and porosity. Data obtained from these techniques have been used to construct models to predict bone mechanical properties thanks to finite element analysis. Cortical porosity has been extensively studied and successfully correlated with disease progression and mechanical properties. Trabecular porosity and pore size distribution, however, have increasingly been taken into consideration to obtain a comprehensive analysis of bone pathology and mechanic. Therefore, we have decided to evaluate the ability of micro- CT (chosen for its high spatial resolving power) and LF-NMR (chosen to analyze the behavior of water molecules within trabecular bone pores) to characterize the morphology of trabecular bone in osteoporosis. Trabecular bone samples from human femoral heads collected during hip replacement surgery were from osteoporosis (test group) and osteoarthritis (control group) patients. Our data show that both micro- CT and LF-NMR can detect qualitative changes in trabecular bone (i.e., transition from plate-like to rod-like morphology). Micro- CT failed to detect significant differences in trabecular bone morphology parameters between osteoporotic and osteoarthritic specimens, with the exception of Trabecular Number and Connectivity Density, which are markers of osteoporosis progression. In contrast, LF-NMR was able to detect significant differences in porosity and pore size of trabecular bone from osteoporotic versus osteoarthritic (control) samples. However, only the combination of these two techniques allowed the detection of structural morphometric changes (increase in the larger pore fraction and enlargement of the larger pores) in the trabecular bone of osteoporotic specimens compared to osteoarthritic ones. In conclusion, the combined use of LF-NMR and micro- CT provides a valuable tool for characterizing the morphology of trabecular bone and may offer the possibility for a new approach to the study and modeling of bone mechanics in the context of aging and disease.

Comparison of Trabecular Bone Mineral Density Measurement Using Hounsfield Unit and Trabecular Microstructure in Orthodontic Patients Using Cone-Beam Computed Tomography

The aim of this study was to measure the bone mineral density of specific regions of maxilla, mandible, and first cervical vertebra using the Hounsfield unit and trabecular microstructure pattern analysis and to compare the two methods. In this study, cone-beam computed tomography (CBCT) images were obtained from 58 patients. Trabecular thickness, trabecular number, trabecular separation, and bone volume fraction were measured in 484 regions for trabecular microstructure parameters and Hounsfield unit was measured for the grayscale value. There was no difference in bone mineral density between the right and left side in every site and between males and females. Trabecular thickness and trabecular number were high in the order of anterior base of the maxilla, mandibular body, first cervical vertebra, and mandibular condyle. Bone volume fraction and Hounsfield unit were high in the order of anterior base of the maxilla, mandibular body, mandibular condyle, and first cervical vertebra (p < 0.05). Trabecular thickness, trabecular number, and bone volume fraction was positively correlated to the Hounsfield unit, and trabecular separation was negatively correlated to the Hounsfield unit (p < 0.005). This study suggests that it is possible to compare the bone mineral density of trabecular bone in various sites using the Hounsfield unit and trabecular microstructure pattern analysis.

Effect of total edentulism on the internal bone structure of mandibular condyle: a preliminary study

OBJECTIVE

The aim of this study was to assess the internal structure of the mandibular condyles in patients with total edentulism by applying fractal analysis on cone-beam computed tomography (CBCT) images and compare with dentate subjects.

METHODS

A total of 136 mandibular condyles were assessed. All CBCT images were prepared with 0.25-mm slice thickness and maximum magnification. A 64 × 64 pixel region of interest (ROI) within the trabecular bone in the center of the condyle was selected. Fractal analysis was performed using White and Rudolph's box-counting method.

RESULTS

Control and study groups were age and gender-matched. The mean fractal dimension (FD) value of the study group was significantly lower compared to the control group (p < 0.01). In the study group, the mean FD value of the females was significantly lower than males (p = 0.01). For the control group, there was no significant difference between the genders for FD (p = 0.11). There was no significant difference between the age subgroups of the female study group and the male control group for FD (p = 0.22, p = 0.33). The mean FD value of the younger age subgroup was significantly higher compared to the older age subgroup in the female control and male study group (p = 0.03, p = 0.01).

CONCLUSION

The internal bone structure of mandibular condyles in edentulous patients was found to be affected by edentulism. Fractal analysis can be used as an effective method for assessing temporomandibular joint health.

Three-dimensional morphometric properties of rod- and plate-like trabeculae in adolescent cancellous bone

Background/Objective

Despite many researches have been carried out on the three-dimensional microarchitecture of cancellous bone, the morphometric properties of rod and plate trabeculae in adolescent cancellous bone have not yet been investigated. This study aimed to investigate three-dimensional morphometric properties of rod- and plate-like trabeculae in normal adolescent cancellous bone, and to compare them with adult cancellous bones to reveal morphometric changes from adolescence to adult life to obtain more insight into the subchondral bone adaptations during development and growth.

Methods

This study included 23 normal human proximal tibiae. These tibiae were divided into three groups: adolescents (9–17 years, n = 6), young adults (18–24 years, n = 9), and adults (25–30 years, n = 8). From each tibia, six cubic cancellous bone samples (dimensions 8 × 8 × 8 mm³) were sawed from each medial and lateral condyle, yielding a total of 276 samples. These samples were scanned using micro computed tomography leading to three-dimensional cubic voxel sizes of 10.5 × 10.5 × 10.5 μm³. The morphometric parameters of individual rod- and plate-like trabeculae were calculated and compared among three age groups.

Results

Significant differences in some morphometric parameters were revealed. The mean longitudinal length of rods was significantly greater in the adolescents than in the young adults. Plate volume density showed an increasing trend with age, although not significant. Trabeculae were more plate-like in adolescents in the medial condyle of adolescents than in the lateral condyle, and changed towards more plate-like trabeculae in the adults. The single best predictor for the mechanical properties was apparent density. Apparent density alone explained 59% variations in Young’s modulus, 77% in ultimate stress and 34% in failure energy, respectively (all p < 0.01). Morphometric parameters might improve this prediction.

Conclusion

In conclusion, this study has reported for the first time the morphometric parameters of rod- and plate-like trabeculae in adolescent proximal tibial cancellous bone, which will improve our understanding of morphometric changes in individual trabeculae during development and growth. Furthermore, separate analysis of individual rods and plates may also help reveal disease-related morphometric changes beyond bone mineral density.

The translational potential of this article

A thorough quantification of individual trabeculae during development and growth may help understand disease-related 3-D morphometric changes beyond bone mineral density.

Structural and Mechanical Properties of Mandibular Condylar Bone

The trabecular bone of the mandibular condyle is structurally anisotropic and heterogeneous. We hypothesized that its apparent elastic moduli are also anisotropic and heterogeneous, and depend on trabecular density and orientation. Eleven condyles were scanned with a micro-CT system. Volumes of interest were selected for the construction of finite element models. We simulated compressive and shear tests to determine the principal mechanical directions and the apparent elastic moduli. Compressive moduli were relatively large in directions acting in the sagittal plane, and small in the mediolateral direction. The degree of mechanical anisotropy ranged from 4.7 to 10.8. Shear moduli were largest in the sagittal plane and smallest in the transverse plane. The magnitudes of the moduli varied with the condylar region and were proportional to the bone volume fraction. Furthermore, principal mechanical direction correlated significantly with principal structural direction. It was concluded that variation in trabecular structure coincides with variation in apparent mechanical properties.

Thrombin receptor deficiency leads to a high bone mass phenotype by decreasing the RANKL/OPG ratio

Thrombin and its receptor (TR) are, respectively, expressed in osteoclasts and osteoblasts. However, their physiological roles on bone metabolism have not been fully elucidated. Here we investigated the bone microarchitecture by micro-computed tomography (μCT) and demonstrated increased trabecular and cortical bone mass in femurs of TR KO mice compared to WT littermates. Trabecular thickness and connectivity were significantly enhanced. The physiological role of TR on both inorganic and organic phases of bone is illustrated by a significant increase in BMD and a decrease in urinary deoxypyridinoline (DPD) crosslink concentration in TR KO mice. Moreover, TR KO cortical bone expanded and had a higher polar moment of inertia (J), implying stronger bone. Bone histomorphometry illustrated unaltered osteoblast and osteoclast number and surface in femoral metaphyses, indicating that thrombin/TR regulates osteoblasts and osteoclasts at functional levels. Serum analysis showed a decrease in RANKL and an increase in osteoprotegerin (OPG) levels and reflected a reduced RANKL/OPG ratio in the TR KO group. In vitro experiments using MC3T3 pre-osteoblasts demonstrated a TR-dependent stimulatory effect of thrombin on the RANKL/OPG ratio. This effect was blocked by TR antagonist and p42/p44-ERK inhibitor. In addition, thrombin also intensified p42/p44-ERK expression and phosphorylation. In conclusion, the thrombin/TR system maintains normal bone remodeling by activating RANKL and limiting OPG synthesis by osteoblasts through the p42/44-ERK signaling pathway. Consequently, TR deficiency inhibits osteoclastogenesis, resulting in a high bone mass phenotype.

A longitudinal study of the effect of experimental osteoporosis on bone trabecular structure in the rat mandibular condyle

The authors performed a longitudinal study of the microstructural changes occurring in the mandibular condyle during osteoporosis using the findings obtained from micro-CT. The subjects used in this study were eight Sprague-Dawley rats. Among them, five were administered the immunosuppressant drug FK506 by injection for five weeks, while the other three were administered saline solution in the same manner. Micro-CT images were taken of the bilateral mandibular condyle, hip, and knee joints in all animals on days 1, 8, 15, 22, 29, and 36 following injection. Six indices of morphometric analysis were compared between the two groups. Significant differences were observed in BV/TV, Tb.Th, Tb.N, and Tb.Sp in the mandibular condyle, while trabecular bone density appeared to decrease in the immunosuppressant group on three-dimensional (3D) imaging. And, in comparison with the mandibular condyle and femur, they were similar. These results suggested that osteoporosis affects not only the femur, but also the mandibular condyle.

Assessment of articular cartilage and subchondral bone using EPIC-microCT in Labrador retrievers with incipient medial coronoid disease

The aetiopathogenesis of medial coronoid disease (MCD) remains obscure, despite its high prevalence. The role of changes to subchondral bone or articular cartilage is much debated. Although there is evidence of micro-damage to subchondral bone, it is not known whether this is a cause or a consequence of MCD, nor is it known whether articular cartilage is modified in the early stages of the disease. The aim of the present study was to use equilibrium partitioning of an ionic contrast agent with micro-computed tomography (microCT) to investigate changes to both the articular cartilage and the subchondral bone of the medial coronoid processes (MCP) of growing Labrador retrievers at an early stage of the disease and at different bodyweights. Of 14 purpose-bred Labrador retrievers (15-27 weeks), six were diagnosed with bilateral MCD and one was diagnosed with unilateral MCD on the basis of microCT studies. The mean X-ray attenuation of articular cartilage was significantly higher in dogs with MCD than in dogs without MCD (P<0.01). In all dogs, the mean X-ray attenuation of articular cartilage was significantly higher at the lateral (P<0.001) than at the proximal aspect of the MCP, indicating decreased glycosaminoglycan content. Changes in parameters of subchondral bone micro-architecture, namely the ratio of bone volume to tissue volume (BV/TV), bone surface density (BS/TV), bone surface to volume ratio (BS/BV), trabecular thickness (Tb.Th; mm), size of marrow cavities described by trabecular spacing (Tb.Sp; mm), and structural model index (SMI), differed significantly by litter (P<0.05) due to the difference in age and weight, but not by the presence/absence of MCD (P>0.05), indicating that subchondral bone density is not affected in early MCD. This study demonstrated that cartilage matrix and not subchondral bone density is affected in the early stages of MCD.

Effects of immediate and delayed loading on peri-implant trabecular structures: a cone beam CT evaluation

PURPOSE

To develop a method for characterizing trabecular bone microarchitecture using cone beam computed tomography (CBCT) and to evaluate trabecular bone changes after rehabilitation using immediate versus delayed implant protocols.

MATERIALS AND METHODS

Six mongrel dogs randomly received 27 titanium implants in the maxillary incisor or mandibular premolar areas, following one of four protocols: (1) normal extraction socket healing; (2) immediate implant placement and immediate loading; (3) delayed implant placement and delayed loading; (4) delayed implant placement and immediate loading. The animals were euthanized at 8 weeks, and block biopsies were scanned using high resolution CBCT. Standard bone structural variables were assessed in coronal, middle, and apical levels.

RESULTS

Coronal and middle regions had more compact, more platelike, and thicker trabeculae. Protocols (2), (3), and (4) had significantly higher values (p < 0.001) than protocol (1) for bone surface density, bone surface volume ratio, and connectivity density, while significantly lower values (p < 0.001) were found for trabecular separation and fractal dimension. However, protocols (2), (3), and (4) did not show significantly different bone remodeling.

CONCLUSIONS

Compared with normal extraction healing, the implant protocols have an improved bone structural integration. Results do not suggest a different bone remodeling pattern when a delayed versus an immediate implant protocol is used.

Preliminary radiological assessment of age-related change in the trabecular structure of the human os pubis

This preliminary study examines correlations between age-at-death and changes in the trabecular architecture of the human os pubis, utilizing continuous, quantitative data from plain film radiography, computed tomography (CT), and micro-CT scans of 65 male innominates. Radiography provides nondestructive options for assessment, digital preservation, and presentation of human skeletal remains; important for forensic and culturally sensitive archaeological materials, which must remain unmodified for opposing experts, future researchers, or repatriation and reburial. Radiographic techniques permit analysis of remains that cannot be disarticulated (e.g., religious proscription, mummies), and trabecular measures provide data where traditional surface indicators are obscured or damaged. Potentially, robust predictive models derived herein achieve R-values of 0.522, 0.447, and 0.731, respectively. Further testing of these methods may validate these techniques as further lines of evidence in age estimation, with the potential to improve on the accuracy of traditional qualitative techniques by providing quantitative, continuous variables in predicting skeletal age-at-death.

Trabecular bone structure in the mandibular condyles of gouging and nongouging platyrrhine primates

The relationship between mandibular form and biomechanical function is a topic of significant interest to morphologists and paleontologists alike. Several previous studies have examined the morphology of the mandible in gouging and nongouging primates as a means of understanding the anatomical correlates of this feeding behavior. The goal of the current study was to quantify the trabecular bone structure of the mandibular condyle of gouging and nongouging primates to assess the functional morphology of the jaw in these animals. High-resolution computed tomography scan data were collected from the mandibles of five adult common marmosets (Callithrix jacchus), saddle-back tamarins (Saguinus fuscicollis), and squirrel monkeys (Saimiri sciureus), respectively, and various three-dimensional morphometric parameters were measured from the condylar trabecular bone. No significant differences were found among the taxa for most trabecular bone structural features. Importantly, no mechanically significant parameters, such as bone volume fraction and degree of anisotropy, were found to vary significantly between gouging and nongouging primates. The lack of significant differences in mechanically relevant structural parameters among these three platyrrhine taxa may suggest that gouging as a habitual dietary behavior does not involve significantly higher loads on the mandibular condyle than other masticatory behaviors. Alternatively, the similarities in trabecular architecture across these three taxa may indicate that trabecular bone is relatively unimportant mechanically in the condyle of these primates and therefore is functionally uninformative.

Type and orientation of yielded trabeculae during overloading of trabecular bone along orthogonal directions

Trabecular architecture plays a major role in bone mechanics. Osteoporosis leads to a transition from a plate-like to a more rod-like trabecular morphology, which may contribute to fracture risk beyond that predicted by changes in density. In this study, microstructural finite element analysis results were analyzed using individual trabeculae segmentation (ITS) to identify the type and orientation of trabeculae where tissue yielded during compressive overloads in two orthogonal directions. For both apparent loading conditions, most of the yielded tissue was found in longitudinally oriented plates. However, the primary loading mode of yielded trabeculae was axial compression with superposed bending for on-axis loading in contrast to bending for transverse loading. For either loading direction, most plate-like trabeculae yielded in the same loading mode, regardless of their orientation. In contrast, rods oriented parallel to the loading axis yielded in compression, while rods oblique or perpendicular to the loading axis yielded in combined bending and tension. The predominance of tissue yielding in plates during both on-axis and transverse overloading explains why on-axis overloading is detrimental to the off-axis mechanical properties. At the same time, a large fraction of the tissue in rod-like trabeculae parallel to the loading direction yielded in both on-axis and transverse loading. Hence, rods may be more likely to be damaged and potentially resorbed by damage mediated remodeling.

Effects of trabecular type and orientation on microdamage susceptibility in trabecular bone

Trabecular architecture becomes more rod-like and anisotropic in osteoporotic and aging trabecular bone. In order to address the effects of trabecular type and orientation on trabecular bone damage mechanics, microstructural finite element modeling was used to identify the yielded tissue in ten bovine tibial trabecular bone samples compressed to 1.2% on-axis apparent strain. The yielded tissue was mapped onto individual trabeculae identified by an Individual Trabeculae Segmentation (ITS) technique, and the distribution of the predicted yielding among trabecular types and orientations was compared to the experimentally measured microdamage. Although most of the predicted yielded tissue was found in longitudinal plates (73+/-11%), the measured microcrack density was positively correlated with the proportion of the yielded tissue in longitudinal rods (R(2)=0.52, p=0.02), but not in rods of other directions or plates. The overall fraction of rods and the fractions of rods along the longitudinal and transverse axes were also correlated with the measured microcrack density. In contrast, diffuse damage area did not correlate with any of these quantities. These results agree with the findings that both in vitro and in vivo microcrack densities are correlated with Structure Model Index (SMI), and are also consistent with decreased energy to failure in more rod-like trabecular bone. Together the results suggest that bending or buckling deformations of rod-like trabeculae may make trabecular structures more susceptible to microdamage formation. Moreover, while simple strain-based tissue yield criteria may account for macroscopic yielding, they may not be suitable for identifying damage.

Modelling Young's modulus for porous bones with microstructural variation and anisotropy

A structural model with three compositional phases and two levels of hierarchical organization is proposed for predicting Young's modulus of porous bones with microstructural variations and anisotropy based on their geometric similarity to metal foams. It has been shown that the proposed single model provides predictions of Young's modulus with high accuracy up to +/-30% for cortical and cancellous bones compared with measured data from the literature. In addition, the conversion of the solid bone shape from "Plate-like" to "Rod-like" at a porosity of 70% or higher (BV/TV 30% or lower)-verified by observations-can be predicted using the proposed model.

Microarchitectural adaptations in aging and osteoarthrotic subchondral bone issues

The human skeleton optimizes its microarchitecture by elaborate adaptations to mechanical loading during development and growth. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction stimulating bone modelling, and remodeling resulting in either bone formation or resorption. This process causes appropriate microarchitectural changes tending to adjust and improve the bone structure to its prevailing mechanical environment. Normal individual reaches peak bone mass at age between 25 and 30 years, and thereafter bone mass declines with age in both genders. The bone loss is accompanied by microarchitectural deterioration resulting in reduced mechanical strength likely leading to fragility fractures. With aging, inevitable bone loss occurs, which is frequently the cause of osteoporosis; and inevitable bone and joint degeneration happens, which often results in osteoarthrosis. These diseases are among the major health care problems in terms of socio-economic costs. The overall goals of the current series of studies were to investigate the age-related and osteoarthrosis (OA) related changes in the 3-D microarchitectural properties, mechanical properties, collagen and mineral quality of subchondral cancellous and cortical bone tissues. The studies included mainly two parts. For human subjects: aging- (I–IV) and early OArelated (V–VI) changes in cancellous bone properties were assessed. For OA guinea pig models (VII–IX), three topics were studied: firstly, the spontaneous, age-related development of guinea pig OA; secondly, the potential effects of hyaluronan on OA subchondral bone tissues; and thirdly, the effects on OA progression of an increase in subchondral bone density by inhibition of bone remodeling with a bisphosphonate. These investigations aimed to obtain more insight into the age-related and OA-related subchondral bone adaptations.

Effects of loading orientation on the morphology of the predicted yielded regions in trabecular bone

While the effects of bone mineral density and architecture in osteoporotic bone have been studied extensively, the micromechanics of yielding and failure have received less attention. However, understanding architectural features associated with failure should provide insight into assessing bone quality. In this study, microstructural finite element models were used to compute regions of tissue level yielding in ten bovine tibial trabecular bone samples. The morphology, number, and mean volume of the yielded regions were quantified for four apparent strains under two loading conditions. For on-axis loading, the mean aspect ratio of the tissue that yielded due to compressive strain increased with increasing apparent strain, expanding along the principal trabecular orientation. This suggests that tissue level yielding progresses along vertical trabeculae when a specimen is loaded on-axis. The number, but not the volume, of the regions yielded due to tensile strain increased with increasing applied load, consistent with relaxation and redistribution of stresses around the yielded regions. When the specimens were compressed perpendicular to the principal axis, the aspect ratio of the yielded regions was close to one, while the number, mean volume, and mean thickness of the yielded regions increased. This indicates that localized high strains consistent with bending rather than axial deformation of struts occur at the tissue level. Overall, the results provide new insight into trabecular bone failure, which is relevant to assessing diagnostic tests for fracture risk or evaluating osteoporosis treatments.

Biomechanical effect of mineral heterogeneity in trabecular bone

Due to daily loading, trabecular bone is subjected to deformations (i.e., strain), which lead to stress in the bone tissue. When stress and/or strain deviate from the normal range, the remodeling process leads to adaptation of the bone architecture and its degree of mineralization to effectively withstand the sustained altered loading. As the apparent mechanical properties of bone are assumed to depend on the degree and distribution of mineralization, the goal of the present study was examine the influences of mineral heterogeneity on the biomechanical properties of trabecular bone in the human mandibular condyle. For this purpose nine right condyles from human dentate mandibles were scanned and evaluated with a microCT system. Cubic regional volumes of interest were defined, and each was transformed into two different types of finite element (FE) models, one homogeneous and one heterogeneous. In the heterogeneous models the element tissue moduli were scaled to the local degree of mineralization, which was determined using microCT. Compression and shear tests were simulated to determine the apparent elastic moduli in both model types. The incorporation of mineralization variation decreased the apparent Young's and shear moduli by maximally 21% in comparison to the homogeneous models. The heterogeneous model apparent moduli correlated significantly with bone volume fraction and degree of mineralization. It was concluded that disregarding mineral heterogeneity may lead to considerable overestimation of apparent elastic moduli in FE models.

Time-lapse observation of rat periodontal ligament during function and tooth movement, using microcomputed tomography

The aim of this study was to observe the time-lapse changes in the rat periodontal ligament (PDL) during function and tooth movement. Under Nembutal anaesthesia, time-lapse changes in the thickness of the PDL of the first molars were investigated in five 12-week-old adolescent rats with microcomputed tomography. Three-dimensional (3D) images were reconstructed from the data. Histological observation was also performed, using undecalcified frozen sections of the maxillary first molar area. The PDL appeared as a radiolucent furrow on the 3D images. A slight change in the thickness of the PDL was observed 1 hour after initiation of orthodontic force loading, which became significant after 6 hours, with the appearance of pressure-tension zones during the tooth movement. These changes were more significant 3 days after orthodontic loading. Histological observation of the lingual cervical PDL (pressure zone) in nine 12- to 13-week-old rats demonstrated that the periodontal space had become narrow and the cellular elements appeared to be densely packed in the narrowed PDL 6 hours after orthodontic loading. Degeneration of tissues appeared 3 days after loading. Observation of the buccal cervical PDL (tension zone) demonstrated that the PDL was extended 6 hours after orthodontic force loading, and the extension continued for up to 3 days. Alkaline phosphatase activity was distributed in the PDL, except for the degenerating tissues in the pressure zone 3 days after loading. The results suggest that the periodontal reaction was initiated within 6 hours after orthodontic force loading, which was related to the structural changes of the PDL. The changes probably induced an early response in individual cells of the PDL.

Effect of food consistency on the degree of mineralization in the rat mandible

A switch to a soft diet, associated with reduced forces applied to the mandible during mastication, may result in an alteration of the degree of mineralization in the mandible. This alteration may be regionally different. The aim of this study was to analyze this alteration by examination of the degree of mineralization in the mandible of growing rats fed with a hard or soft diet. Fifteen Wistar male rats were used in this investigation. After weaning, six rats were fed with a hard diet and the remaining nine rats with a soft diet. After 9 weeks, three-dimensional reconstructions of the cortical and trabecular bone of their mandibles were obtained using a microCT system. The degree of mineralization was determined for the trabecular bone in the condyle and for the cortical bone in the anterior and posterior areas of the mandibular body. In both diet groups the degree of mineralization was significantly (p < 0.01) lower in the trabecular than in the cortical bone. In the mandibular body, the anterior area showed a significantly (p < 0.01) higher degree of mineralization than the posterior area in both diet groups. In both areas the soft diet group had a significantly (p < 0.05 or 0.01) higher degree of mineralization than the hard diet group. The trabecular bone in the condyle of the hard diet group showed a significantly (p < 0.01) higher degree of mineralization than in the soft diet group. The present results indicate the importance of proper masticatory muscle function for craniofacial growth and development.

MicroCT examination of human bone specimens: effects of polymethylmethacrylate embedding on structural parameters

X-ray microtomography permits the nondestructive investigation of trabecular and cortical bone specimens without special preparation of the sample. To do a quantitative characterization, the cross-section images have to be binarized, separating bone from nonbone. For this purpose, a widely used method is uniform thresholding. However, for commonly available microtomography scanners which use a polychromatic X-ray source, it is unclear what effect the surrounding medium (e.g. air, saline solution, polymethylmethacrylate) has on the threshold value used for the binarization. In the literature an easy procedure to find the optimal uniform threshold value for a given acquisition condition is reported. By applying this procedure, the present work investigated whether a microtomography scan of trabecular bone samples in air or embedded in polymethylmethacrylate gave the same results in terms of structural parameters. The gold standard, that is, histological sections, was used as a reference. Two fixed threshold values were found, one for the microtomography scans performed in air and one for the scans with the same samples embedded in polymethylmethacrylate. These were applied on the correspondent microtomography images for the estimation of structural parameters, such as bone volume fraction, direct trabecular thickness, direct trabecular separation and structure model index. Paired comparisons were made in bone volume fraction between histological sections and microtomography cross-sections for the same bone samples scanned first in air and then embedded in polymethylmethacrylate, by which no significant differences were found. Paired comparisons were also made in bone volume fraction, direct trabecular thickness, direct trabecular separation and structure model index for the same samples over volumes of interest of 4 x 4 x 4 mm3 between microtomography scans in air and scans with the samples embedded in polymethylmethacrylate. Neither these comparisons showed significant differences. This leads to the conclusion that structural parameters estimated by microtomography for human trabecular bone samples scanned either in air or embedded in polymethylmethacrylate are not affected by the surrounding medium (i.e. presence or absence of polymethylmethacrylate), provided that the corresponding optimal threshold value is applied for each acquisition condition.

Variations in mineralization affect the stress and strain distributions in cortical and trabecular bone

The mechanical properties of bone depend largely on its degree and distribution of mineralization. The present study analyzes the effect of an inhomogeneous distribution of mineralization on the stress and strain distributions in the human mandibular condyle during static clenching. A condyle was scanned with a micro-CT scanner to create a finite element model. For every voxel the degree of mineralization (DMB) was determined from the micro-CT scan. The Young's moduli of the elements were calculated from the DMB using constant, linear, and cubic relations, respectively. Stresses, strains, and displacements in cortical and trabecular bone, as well as the condylar deformation (extension along the antero-posterion axis) and compliance were compared. Over 90% of the bone mineral was located in the cortical bone. The DMB showed large variations in both cortical bone (mean: 884, SD: 111 mg/cm(3)) and trabecular bone (mean: 738, SD: 101 mg/cm(3)). Variations of the stresses and the strains were small in cortical bone, but large in trabecular bone. In the cortical bone an inhomogeneous mineral distribution increased the stresses and the strains. In the trabecular bone, however, it decreased the stresses and increased the strains. Furthermore, the condylar compliance remained relatively constant, but the condylar deformation doubled. It was concluded that neglect of the inhomogeneity of the mineral distribution results in a large underestimation of the stresses and strains of possibly more than 50%. The stiffness of trabecular bone strongly influences the condylar deformation. Vice versa, the condylar deformation largely determines the magnitude of the strains in the trabecular bone.

Degree and distribution of mineralization in the human mandibular condyle

The degree of mineralization of bone (DMB) in the mandibular condyle reflects the age and remodeling rate of the bone tissue. Quantification of DMB facilitates a better understanding of possible effects of adaptive remodeling on mineralization of the condyle and its possible consequences for its mechanical quality. We hypothesized differences in the degree and distribution of mineralization between trabecular and cortical bone and between various cortical regions. Microcomputed tomography was used to measure mineralization in 10 human mandibular condyles. Mean DMB was higher in cortical (1,045 mg hydroxyapatite/cm(3)) than in trabecular bone (857 mg/cm(3)) and differed significantly between cortical regions (anterior 987 mg/cm(3), posterior 1,028 mg/cm(3), subchondral 1,120 mg/cm(3)). The variation of DMB distribution was significantly larger in the anterior cortex than in the posterior and subchondral cortex, indicating a larger amount of heterogeneity of mineralization anteriorly. Within the cortical bone, DMB increased with the distance from the cortical canals to the periphery. Similarly, the DMB of trabecular bone increased with the distance from the surface of the trabeculae to their cores. It was concluded that the rate of remodeling differs between condylar trabecular and cortical bone and between cortical regions and that DMB is not randomly distributed across the bone. The difference in DMB between condylar cortical and trabecular bone suggests a large difference in Young's modulus.